Late last year, Caltech researchers revealed that they had developed a new fabrication technique for printing microsized metal parts containing features about as thick as three or four sheets of paper.

Now, the team has reinvented the technique to allow for printing objects a thousand times smaller: 150 nanometers, which is comparable to the size of a flu virus. In doing so, the team also discovered that the atomic arrangements within these objects are disordered, which would, at large scale, make these materials unusable because they would be considered weak and "low quality." In the case of nanosized metal objects, however, this atomic-level mess has the opposite effect: these parts can be three-to-five-times stronger than similarly sized structures with more orderly atomic arrangements.

The work was conducted in the lab of Julia R. Greer, the Ruben F. and Donna Mettler Professor of Materials Science, Mechanics and Medical Engineering; and Fletcher Jones Foundation Director of the Kavli Nanoscience Institute. The paper describing the work, "Suppressed Size Effect in Nanopillars with Hierarchical Microstructures Enabled by Nanoscale Additive Manufacturing," is published in the August issue of Nano Letters.

The new technique is similar to another announced by the team last year, but with each step of the process reimagined to work at the nanoscale. However, this presents an additional challenge: the manufactured objects are not visible to the naked eye or easily manipulatable.

The process starts with preparing a photosensitive "cocktail" that is largely comprised of a hydrogel, a kind of polymer that can absorb many times its own weight in water. This cocktail is then selectively hardened with a laser to build a 3D scaffold in the same shape as the desired metal objects. In this research, those objects were a series of tiny pillars and nanolattices.

The hydrogel parts are then infused with an aqueous solution containing nickel ions. Once the parts are saturated with metal ions, they are baked until all the hydrogel is burned out, leaving parts in the same shape as the original, though shrunken, and consisting entirely of metal ions that are now oxidized (bound to oxygen atoms). In the final step, the oxygen atoms are chemically stripped out of the parts, converting the metal oxide back into a metallic form.